1. Technical Field of the Invention
The present invention relates to an apparatus and method for processing the components that constitute a neutron lens for converging or diverging a neutron beam.
2. Prior Art
A neutron beam has the following features which are different from an X-ray or a photon, (1) strong mutual reaction with atomic nuclei, (2) similar energy and wavelength thereof to those of motions or structures at the atomic level, (3) retention of a magnetic moment, (4) intense penetrating power, etc. Therefore, when the position of an atomic nucleus is to be studied, for instance, to have information about the position of a hydrogen atom in an organic material, which is very difficult to measure by an X-ray diffraction method, a diffraction experiment using a neutron beam is indispensable. In addition, because the spin of a neutron is xc2xd with a magnetic moment, the magnetic structure of a substance can be investigated conveniently. Furthermore, when the interior of a large object such as an industrial product is to be studied using radio active rays, because a neutron beam has a high penetrating power, fluoroscopy can be used.
However, because neutron beams cannot be generated easily, the sites are limited to nuclear reactors, accelerator facilities, etc. Consequently, a neutron beam must be guided efficiently from the neutron source to the application device in order to irradiate a small sample with a high-density neutron beam. For this purpose, it is essential to have a technology such that the neutron beams can be made parallel and then to make the beams converge sharply.
Recently, the aforementioned technique using neutron beams has been attracting wide attention for analysis etc., and the same applicant as for the present invention has proposed an element for converging or diverging a neutron beam (Japanese patent application No. 60630/1999, not published). In the following paragraphs, this element is called a xe2x80x9cneutron lens.xe2x80x9d
FIG. 1 shows the principle of refraction of a neutron beam by a substance. Mutual reaction between the neutron and the substance occurs mostly with the atomic nuclei contained in the substance and as a result of this reaction, incident neutrons lose part of their kinetic energy when they enter the substance, and the neutrons are slowed down tangentially and normally to the surface boundary of the substance. Therefore, as shown in FIG. 1, a neutron beam entering obliquely through the boundary surface of the substance is refracted with a refractive index of less than 1. At this time, substances which are known to have a refractive index of less than 1 for a neutral beam include O, C, Be and Fe among those with naturally occurring isotopic concentrations, and deuterium D among separated isotopes.
FIG. 2 shows the principles of a neutron lens. This figure illustrates the condition in which a beam of neutrons (beam 16) is incident to a sheet-like member 11. On the surface of the sheet-like member 11 straight protrusions 12 are formed each of which is composed of a substantially vertical surface 14 and an inclined surface 15. A neutron beam 16 entering the inclined surface 15 of the straight protrusion 12 is refracted with an index smaller than 1 as shown in FIG. 1. However, the angle xcex4 of a single refraction is so small that, for instance, when the sheet-like member is composed of polytetrafluoroethylene (PTFE) with a high neutron transmission rate and the inclined surface 15 of the straight protrusion 12 makes an angle xcex1 of 45xc2x0 to the surface of the sheet-like member 11, the angle of refraction xcex4 of a neutron beam with a wavelength of 14 xc3x85 impinging vertically onto the sheet-like member 11 is only 0.14 mrad.
FIG. 3 is an isometric view of a neutron lens capable of converging neutron beams, and FIG. 4 shows the section through the line Axe2x80x94A of the lens. The neutron lens is composed of a main portion 20 and upper and lower ring-shaped outer frames 21, 22 that hold the main portion. The neutron lens is assembled by fastening screws 24 into pins arranged between the two ring-like outer frames 21, 22 hat sandwich the main portion 20.
FIGS. 5A and 5B show the structure of the sheet-like components of the main portion 20. The main portion 20 is constructed by laminating a number of sheet-like components 25 each of which is provided with a hole 32 at the center thereof. The closer the sheet-like component is to the top, the larger is the hole bored in the center, and there is no hole in the center of the bottom sheet-like component. Therefore, the main portion is shaped like an earthenware mortar, that is, the center is a concave cone shape. In the example shown in FIG. 4, there are 33 sheet-like components 25 laminated together. Reference numbers 33a to 33d indicate holes for the pins 23.
In FIGS. 5A and 5B, the sheet-like component 25 is composed of a thin sheet with ring-shaped protrusions 31 which have a triangular shape in section, formed coaxially and continuously in the radial direction. The inclined surface 3A of a ring-shaped protrusion 31 with a triangular shape in section, forms an incident surface inclined to the axis of the incoming neutron beam, and the rings face inwards in coaxial circles, that is, towards the center line of the neutron lens.
Neutron beams, traveling in a direction parallel to the axis of the neutron lens, shown in FIGS. 4, 5A and 5B, enter through the inclined surfaces of the ring-shaped protrusions 31 formed on each sheet-like component, therefore the beams are deflected towards the center line of the neutron lens. Neutron beams entering near the center line are deflected through smaller angles because the beams pass through a relatively small number of ring-shaped protrusions, however, neutron beams entering near the outer periphery are deflected more as the beams penetrate a larger number of ring-shaped protrusions. Consequently, this neutron lens performs a similar function to that of a convex lens in an optical system, and can concentrate the neutron beams into a small area.
If the inclined surfaces 31a of the ring-shaped protrusions 31 are made in outward facing concentric circles in the opposite way to FIGS. 5A and 5B, the neutron lens can function as a concave lens does in an optical system with the same configuration as shown in FIG. 4, thereby neutron beams can be made to diverge.
The sheet-like component 25 should be formed using a substance that has a refractive index of less than 1 for a neutron beam as described above. In the case of elements with naturally occurring isotopic compositions these are substances including the elements O, C, Be and F, and deuterium D in the case of enriched isotopes. Practical materials for these substances are the aforementioned polytetrafluoroethylene (PTFE), graphite, neutron-modified polyethylene wherein the hydrogen is changed to deuterium, etc.
Of these materials, graphite (hereinafter simply called as carbon) is readily available at a rather low cost, therefore, it is required that the above-mentioned sheet-like components should be formed from carbon plates.
However, carbon has the problem that because of its hardness and brittleness, it cannot be machined into the preferred shape by a conventional means of processing, for instance, by cutting, as the edge of the ring-shaped protrusion 31 becomes chipped. Furthermore, a large number of sheet-like components 25 must be stacked together to produce a neutron lens, therefore the thinner the sheet-like components 25, the better it is to make the neutron lens small, that is, it is desirable to make the sheets as thin as about 1 mm. However, a thin carbon sheet suffers from the problem that it is damaged even by the small machining force caused by machining. In addition, to precisely deflect neutron beams, the inclined surfaces 31a of the ring-shaped protrusions 31 should be made very accurately. In addition, to transmit the neutron beams with minimum losses, irregular reflections at the surface must be suppressed so the inclined surfaces 31a must be processed with a superior finish, nearly like a mirror.
The present invention aims to solve these problems. More explicitly, an object of the present invention is to provide a processing apparatus and method for a neutron lens component that can form fine ring-shaped protrusions accurately with a low processing force and excellent surface finish on the surface of a neutron lens component made of graphite etc.
The apparatus provided by the present invention to machine neutron lens components is composed of a rotating table (2) that supports a flat neutron lens component (1) made of a material with a refractive index of less than 1 for a neutron beam and rotates about an axis Z orthogonal to the surface of the aforementioned component, a metal-bonded grinding wheel (3) shaped like a circular disk with one tapering surface (3a) or a plurality of tapering surfaces (3a) on the outer periphery thereof, a grinding wheel driving device (4) that drives the above-mentioned grinding wheel so as to make it rotate around the axis thereof and moves the grinding wheel relative to the rotating table, an electrode (5) located with a surface close to the aforementioned one or several tapering surfaces of the grinding wheel, a power source (6) for applying an electrolytic voltage between the grinding wheel and the electrode, and a grinding fluid feeder (8) for supplying a conducting grinding fluid between the grinding wheel and the electrode, wherein the angle between the aforementioned one or several tapering surfaces (3a) of the grinding wheel is made smaller than the angle of the V-shaped groove produced on the surface of the neutron lens component (1), the above-mentioned grinding wheel driving device (4) is positioned with the axis of the grinding wheel at an oblique angle to the axis of rotation of the neutron lens component, and the inclination thereof can be varied through a small angle about that position.
According to a preferred embodiment of the present invention, the above-mentioned metal-bonded grinding wheel (3) is a cobalt-based bonded grinding wheel containing ultra-fine grinding grains with a mean grain diameter of 10 xcexcm or less.
According to the present invention, (A) the flat neutron lens component (1) made of a material with a refractive index of less than 1 for a neutron beam is attracted to and held on the surface of the rotating table (2) and rotated around the axis Z orthogonal to the surface of the component, (B) the circular-disk metal-bonded grinding wheel (3) with one or several tapering surfaces (3a) shaped with an angle more acute than the angle of the V-shaped groove being formed on the surface of the neutron lens component is driven so as to rotate about the axis thereof and the aforementioned grinding wheel is moved relative to the rotating table, (C) the conducting grinding fluid is fed between the electrode (5) the surface of which is close to the aforementioned one or several tapering surfaces of the grinding wheel and the grinding wheel, at the same time an electrolytic voltage is applied to the electrode and the tapering surfaces are dressed electrolytically, and (D) at the same time, the center line of the grinding wheel is positioned at an oblique angle to the rotating shaft of the neutron lens component, and the oblique angle is varied slightly about that position, thereby the preferred V-like groove is ground and processed on the surface of the neutron lens component using both sides of the tapering surface.
In the above-mentioned apparatus and method of the present invention, the angle formed by one tapering surface (3a) or a plurality of tapering surfaces (3a) of the grinding wheel is shaped with a more acute angle than the angle of the V-shaped groove formed on the surface of the neutron lens component (1), therefore by slightly varying with an oscillating movement, the angle at which the axis of the grinding wheel is inclined to the cutting surface of the neutron lens component, a V-shaped groove with a freely selected angle greater than the angle of the tapering surfaces (3a) can be machined on the surface of the neutron lens component using both sides of the tapering surface. In addition, since the axis of the grinding wheel is at an oblique angle to the axis of rotation of the neutron lens component, both surfaces of the V-shaped groove formed on the surface of the neutron lens component can be machined at a freely selected angle (for instance, one side perpendicular to the surface, and the other at about 45xc2x0
In addition, because the electrode (5) is provided, the surface of which is located close to the tapering surface of the grinding wheel, and while a conducting grinding fluid is fed between the electrode and the grinding wheel, a voltage is applied to electrolytically dress the tapering surface, therefore, even if a metal-bonded bonded grinding wheel containing ultra-fine grinding grains with a mean grain diameter of 10 xcexcm or less is used, the grinding, grains can be sharpened and always kept in the optimum sharpened condition, so fine ring-shaped protrusions (V-shaped grooves) can be created accurately with an excellent surface finish and a low machining force.
Other objectives and advantages of the present invention are described below referring to the attached drawings.